X-ray generators of small or moderate power for medical radiological application, normally use a fixed-anode x-ray tube (verses a rotating-anode x-ray tube as used when large power is required). In this case, the x-ray tube is usually contained in the same oil-filled housing as the high-voltage transformer and other components of the high-voltage circuit, and such an assembly is called a tubehead. During the last several years, x-ray generators commercially available for dental application (whether intraoral, panoramic, or other) have adopted this general design almost universally.
Inside the tubehead, the x-ray tube is supported by a mechanical part known as the tube holder, made out of a high-insulate and high electric tensile strength material, which performs essentially two functions:
1) to securely and precisely hold the x-ray tube in position, in relation to the surrounding construction and in particular to the output windows and the external Beam-Limiting-Device; it ensures the accurate geometrical position of the x-ray source;
2) to generate high-voltage insulation between the x-ray tube (one or more of whose electrodes are at extremely high electrical potential) and the surrounding constructive metallic parts (in particular the housing) which are grounded.
In order to provide near-focus shielding against radiation from the x-ray tube (primary and extrafocal) in all directions except through a suitable output windows (thus greatly reducing the additional radiation shielding required for the housing as a whole), the tube holder is usually surrounded by a lead jacket, at least at the anode side and except for a small opening in correspondence with the wanted x-ray beam path.
The design and construction of this lead jacket may be critical, as any sharp or pointed detail (e.g., such as the thread of a screw) should be avoided because they imply singularities in the electric field and hence may cause high-voltage discharges.
Other structural components of the tube head, such as the housing or other such components, have typically been formulated from steel sheet metal, or cast aluminum alloys. Fabrication with welded steel sheet metal is relatively expensive, and quality might be difficult to control because it depends on the accuracy of the individual manufacturing process. Cast aluminum alloys requires special precautions to prevent oil leakage due to the fact that the material is often porous.
In both cases, but particularly when using aluminum, additional x-ray shielding is necessary. Lead plates or foil normally provide this shielding. Lead is an undesirable material to work with because of environmental and health issue concerns. In addition, lead shielding can either be placed inside the housing or outside the housing. In the case of inside shielding, the efficacy of the shielding cannot be visually inspected over time to check for example, for positioning of the lead plates. Further, the lead and its associated components, such as lead protective paints, are potential contaminants to the dielectric-oil, and potentially can spoil the insulating performance of the oil. When placed outside the housing, the shielding may be mechanically damaged (lead is a soft material) by improper handling during production, installation or service. Further, the harmful lead is exposed. A need exists therefore for a dental x-ray apparatus having improved shielding and structural components.